Handle positional args interspersed in the named args

[cgay]

Low priority, I'd say...

Some parsers (Python optparse, possibly the Go flag package) extract positional arguments that are interspersed in between named arguments, and we should probably do the same. Example:

The command line doit -a --when=now dog --why=because cat should be parsed as having positional args #("dog", "cat").

Maybe check C++ and Java arg parsers to see how consistently this is done.